Entanglement change of mixed states under canonical unitary operations in two qubits

We investigate entanglement change of mixed states in a class by applying canonical unitary form of two-qubit unitary operations. The class, including Werner states, Bell diagonal states and maximum entangled mixed states, is characterized by three real parameters and geometrically presented by a tetrahedron. We propose a class of mixed states whose entanglement cannot be increased by any canonical unitary operation.